1. Field of the Invention
The present invention relates to an ink jet recording head, and more particularly to a method for producing a heat-generating substrate for an ink jet recording head adapted for effecting recording by ink discharge from a discharge opening by growth and contraction of a bubble generated in the ink by a discharge energy generating element, such recording head and a recording apparatus utilizing such recording head.
2. Related Background Art
The ink jet recording method described in the U.S. Pat. No. 4,723,129 or No. 4,740,796 is recently attracting particular attention as it is capable of image recording with a high definition and high image quality at a high speed and a high density, and is also suitable for color image recording and for compactization of the apparatus. In a representative configuration of the recording apparatus employing such method, there is provided a heat action area for applying heat to the recording liquid or the like (hereinafter called ink) in order to discharge the ink by thermal energy. More detailedly, corresponding to an ink flow path, there is provided an electro-thermal converting element including a pair of connecting electrodes and a heat-generating resistance layer connected between said electrodes and adapted to generate heat in the area between the electrodes, and the thermal energy generated from said heat-generating resistance layer is utilized for rapidly heating the ink on the heat action area to generate bubble whereby the ink is discharged by such bubble generation.
Since such heat action area of the ink jet recording head is exposed to severe conditions including mechanical impact and erosion resulting from cavitation caused by repeated bubble generation and extinction in the ink and temperature ascent and descent of about 1000.degree. C. within an extremely short time of 0.1 to 10 microseconds, there is provided a protective film for protecting the heat-generating resistance layer from such harsh conditions. Such protective film is required to be excellent in heat resistance, liquid resistance, resistance to liquid permeation, stability against oxidation, electric insulation, breakage resistance and thermal conductivity, and is generally composed of an inorganic compound such as SiO or SiN. Also a single-layered protective film may not be sufficient for protecting the heat-generating resistance layer, and a metallic film of higher anticavitation property, composed for example of Ta, may be provided on the protective film.
The above-explained configuration is employed not only on the heat-generating resistance layer but also on the wiring patterns for electric connection with the heat-generating resistance layer, in order to prevent corrosion of the wirings by the ink.
FIG. 3 is a schematic plan view of a part of the substrate for a conventional ink jet recording head, and FIG. 4 is a partial cross-sectional view of said substrate along a chain line 4--4, in FIG. 3.
Referring to FIGS. 3 and 4, a Si substrate 120 is provided thereon with a heat accumulating layer 106 composed of SiO.sub.2, formed for example by thermal oxidation. On said substrate 120 with the heat accumulating layer 106, there are formed a heat-generating resistance layer 107 for applying thermal energy to the ink, and wirings 103, 104 for applying a voltage to said heat-generating resistance layer. A part of the heat-generating resistance layer 107, exposed from the wirings 103, 104 constitutes a heat-generating portion 102. On said heat-generating resistance layer and wirings, there are provided an insulating protective film 108 and an anticavltation Ta film 110.
In the ink jet recording head, the heat-generating substrate constituting the heat action area is constructed as explained above, and the structure of the protective film mentioned above is an important factor determining the performance of the ink jet recording head, such as the electric power consumption and the service life thereof.
However, in the conventional configuration of the protective film, the reduction in electric power consumption is a trade-off to the improvement in film reliability and service life.
For example, the electric power required for bubble generation can be reduced as the film between the heat-generating resistance and the ink becomes thinner or has a higher thermal conductivity, since heat dissipation other than to the ink can be reduced. Stated differently, the efficiency of energy can be improved as the protective film becomes thinner.
On the other hand, a thinner protective film is apt to form pinholes thereon or to be unable to sufficiently cover the stepped portion of the wiring, resulting in defective coverage on such stepped portion. Such defective coverage results in ink intrusion, thus leading to erosion of the wiring and the heat-generating resistance and deterioration in the reliability and in the service life.
In consideration of the foregoing, the Japanese Patent Laid-open Application No. 62-103148 discloses a configuration of forming the protective film thinner only in a portion thereof involved in the bubble generation, thereby reducing the electric power consumption while improving the film reliability and the service life.
However, in the above-mentioned patent, dry half etching is suggested for forming the thinner portion of the protective film, but the film thickness is difficult to control with such method because the film thickness is principally controlled by the etching time in this method.
On the other hand, on the heat-generating portion in the ink jet recording head, the protective film is required to have a uniform thickness, since, if the protective film on the heat-generating portion is uneven in thickness, the center of bubble generation may be displaced from the center of the heat-generating resistor or the bubble generating characteristics may be altered to affect the ink discharge characteristics.
As explained in the foregoing, the conventional configuration is apt to cause fluctuation in the thickness of the protective film on the heat generating portion in the recording head, so that uniform discharge characteristics are difficult to obtain among different discharge openings and there may result deterioration in the print quality.